PROJECT SUMMARY Insertional Achilles tendinopathy (IAT) is a debilitating disorder that responds poorly to conservative (non- surgical) therapies. An effective conservative treatment for this disease must target the fundamental causes of pathological tissue alterations and induce deformations that promote their reversal. Thus, the objective of this project is to elucidate the patterns of mechanical strain (i.e., deformation) that cause and reverse IAT in vitro, and determine how to induce these strain patterns in vivo during exercise-based physical therapy. In preliminary work, we have used ultrasound elastography to demonstrate that IAT-associated changes are greater in the deep tendon, where transverse compressive strain |?2?| ? compressive strain along the short-axis of the tendon ? is highest due to contact with the heel bone. Moreover, we have determined that tendon explants loaded under transverse compression in vitro exhibit alterations that resemble IAT. Importantly, previous studies have demonstrated that transverse compression-induced tendon alterations can be reversed by axial tensile strain |?1?| (strain along the long axis of the tendon). These findings support our central hypothesis that deformations with high ratios |??|/|??| of transverse compressive strain to axial tensile strain 21 cause IAT while deformations with low strain ratios |??|/|??| can reverse this disease. To test this hypothesis, 21 in Aim 1 we propose to investigate whether high levels of |??|/|??| generate IAT-like changes in porcine 21 Achilles tendon insertion explants in vitro. In Aim 2, we will investigate whether low levels of |??|/|??| can 21 reverse IAT-associated changes in human IAT tendon explants in vitro. In Aims 1-2, we will also test the secondary hypothesis that the differential response of tendon to |?G| and |?G| is due to their markedly different 21 effects on tendon cell nuclear volume and shape, a concept supported by preliminary studies. These effects could lead to distinct alterations in nuclear chromatin organization or nuclear transport that mediate gene expression and tissue composition. Finally, in Aim 3, we will characterize the mechanical strain patterns induced by different exercises in vivo and establish how exercises can be adjusted to modulate levels of |??|/|??| in the Achilles tendon insertion. This study will take a significant step towards establishing effective, 21 targeted conservative treatment for patients with IAT. Moreover, the findings and approaches established in this proposal could also be applicable to other important tendon and ligament diseases including rotator cuff disease, carpal tunnel syndrome and patellar tendinopathy.